feat: allow building on other compatible architectures

craft_application/_config.py

@@ -43,6 +43,13 @@ class ConfigModel(pydantic.BaseModel):
     """The platform for which to build."""
     build_for: str | None = None
     """The target architecture for which to build."""
+    build_on: str | None = None
+    """The architecture on which to build.
+
+    This is ignored in destructive mode and is only used for launching a provider.
+    The provider is instructed about the requested architecture. If that architecture
+    cannot run with the current system configuration, it will raise an error.
+    """
 
     parallel_build_count: int
     """The parallel build count to send to Craft Parts.

craft_application/services/provider.py

@@ -215,6 +215,8 @@ def instance(
         if clean_existing:
             self._clean_instance(provider, work_dir, build_info, project_name)
 
+        build_on = self._services.get("config").get("build_on")
+
         emit.progress(f"Launching managed {base_name[0]} {base_name[1]} instance...")
         with provider.launched_environment(
             project_name=project_name,
@@ -225,6 +227,7 @@ def instance(
             use_base_instance=use_base_instance,
             prepare_instance=prepare_instance,
             shutdown_delay_mins=shutdown_delay,
+            instance_architecture=build_on,
         ) as instance:
             instance.mount(
                 host_source=work_dir,

docs/reference/changelog.rst

@@ -18,7 +18,16 @@ Changelog
 6.3.0 (unreleased)
 ------------------
 
-Testing
+Providers
+=========
+
+- Craft Application can now build on a provider using a different architecture
+  from the host architecture as long as the provider supports that architecture.
+  Set the ``CRAFT_BUILD_ON`` environment variable to an architecture to use
+  this.
+
+Commands
+========
 
 - The ``test`` command now reads ``spread.yaml`` before beginning the lifecycle,
   erroring early if the file is invalid.

pyproject.toml

@@ -9,7 +9,7 @@ dependencies = [
     "craft-grammar>=2.3.0",
     "craft-parts>=2.28.0",
     "craft-platforms>=0.6.0",
-    "craft-providers>=3.3.0",
+    "craft-providers>=3.5.0",
     "Jinja2>=3.1.6,<4.0.0",
     "snap-helpers>=0.4.2",
     "platformdirs>=3.10",

spread.yaml

@@ -33,6 +33,12 @@ backends:
           workers: 4
           memory: 4G
           storage: 20G
+      # - ubuntu-24.04-arm-64:
+      #     image: ubuntu-os-cloud/ubuntu-2404-lts-arm64
+      #     workers: 4
+      #     memory: 4G
+      #     storage: 20G
+
   # lxd:  # Disabled due to https://github.com/canonical/spread/issues/215
   #   systems:
   #     - ubuntu-24.04
@@ -143,7 +149,7 @@ prepare: |
   sudo snap install snapd || sudo snap refresh snapd
   sudo snap wait system seed.loaded
 
-  sudo snap install --dangerous --classic tests/spread/*.snap
+  sudo snap install --dangerous --classic tests/spread/*$(dpkg --print-architecture).snap
 
   # Select older LXD channels for older bases.
   . /etc/os-release

tests/integration/services/test_provider.py

@@ -16,11 +16,13 @@
 """Integration tests for provider service."""
 
 import contextlib
+import pathlib
 import subprocess
 
 import craft_platforms
 import craft_providers
 import pytest
+from craft_application import ProviderService
 
 
 @pytest.mark.parametrize(
@@ -142,3 +144,89 @@ def test_run_managed(provider_service, fake_services, fetch, snap_safe_tmp_path)
     provider_service.run_managed(
         build_info, enable_fetch_service=fetch, command=["echo", "hi"]
     )
+
+
+@pytest.mark.slow
+@pytest.mark.parametrize(
+    "build_on",
+    [
+        pytest.param(
+            "riscv64",
+            marks=pytest.mark.skipif(
+                craft_platforms.DebianArchitecture.from_host()
+                == craft_platforms.DebianArchitecture.RISCV64,
+                reason="Not testing on compatible host.",
+            ),
+        ),
+        pytest.param(
+            "s390x",
+            marks=pytest.mark.skipif(
+                craft_platforms.DebianArchitecture.from_host()
+                == craft_platforms.DebianArchitecture.S390X,
+                reason="Not testing on compatible host.",
+            ),
+        ),
+    ],
+)
+@pytest.mark.parametrize("base", [craft_platforms.DistroBase("ubuntu", "26.04")])
+def test_get_incompatible_instance_error(
+    monkeypatch: pytest.MonkeyPatch,
+    tmp_path: pathlib.Path,
+    provider_service: ProviderService,
+    build_on: str,
+    base: craft_platforms.DistroBase,
+):
+    monkeypatch.setenv("CRAFT_BUILD_ON", build_on)
+
+    build_info = craft_platforms.BuildInfo(
+        platform="foo",
+        build_on=craft_platforms.DebianArchitecture(build_on),
+        build_for=craft_platforms.DebianArchitecture.from_host(),
+        build_base=base,
+    )
+
+    with pytest.raises(
+        craft_providers.errors.ProviderError,
+        match="Requested architecture isn't supported by this host",
+    ):
+        with provider_service.instance(build_info, work_dir=tmp_path):
+            pass
+
+
+@pytest.mark.slow
+@pytest.mark.parametrize(
+    "build_on",
+    [
+        pytest.param(
+            "armhf",
+            marks=pytest.mark.skipif(
+                craft_platforms.DebianArchitecture.from_host()
+                != craft_platforms.DebianArchitecture.ARM64,
+                reason="Skipping incompatible host.",
+            ),
+        ),
+    ],
+)
+@pytest.mark.parametrize("base", [craft_platforms.DistroBase("ubuntu", "26.04")])
+def test_instance_with_different_architecture(
+    monkeypatch: pytest.MonkeyPatch,
+    tmp_path: pathlib.Path,
+    provider_service: ProviderService,
+    build_on: str,
+    base: craft_platforms.DistroBase,
+):
+    monkeypatch.setenv("CRAFT_BUILD_ON", build_on)
+
+    build_info = craft_platforms.BuildInfo(
+        platform="foo",
+        build_on=craft_platforms.DebianArchitecture(build_on),
+        build_for=craft_platforms.DebianArchitecture.from_host(),
+        build_base=base,
+    )
+
+    with provider_service.instance(build_info, work_dir=tmp_path) as instance:
+        result = instance.execute_run(
+            ["dpkg", "--print-architecture"], text=True, capture_output=True
+        )
+
+    assert result.stdout.rstrip() == build_on

tests/spread/testcraft/different-architecture/task.yaml

@@ -0,0 +1,17 @@
+summary: Testcraft can build on armhf on an arm64 runner
+
+priority: 100
+
+systems:
+  - ubuntu-24.04-arm-64
+
+execute: |
+  # Configure testcraft to build on armhf
+  export CRAFT_BUILD_ON=armhf
+
+  testcraft pack --verbosity=debug
+  test -f built-on-armhf-0.1-platform-independent.testcraft
+
+restore: |
+  testcraft clean
+  rm -f built-on-armhf*.testcraft

tests/spread/testcraft/different-architecture/testcraft.yaml

@@ -0,0 +1,16 @@
+name: built-on-armhf
+summary: A testcraft package that gets built on an armhf runner.
+version: "0.1"
+
+base: ubuntu@24.04
+platforms:
+  platform-independent:
+    build-on: [armhf]
+    build-for: [all]
+
+parts:
+  my-test:
+    plugin: nil
+    override-build: |
+      echo -e "#!/bin/bash\necho Hello, world!" > $CRAFT_PART_INSTALL/hello
+      chmod a+x $CRAFT_PART_INSTALL/hello

tests/unit/services/test_provider.py

@@ -663,8 +663,10 @@ def test_instance(
     allow_unstable,
     mock_provider,
 ):
-    # In case the user's system has this set.
+    # In case the user's system has these set.
     monkeypatch.delenv("CRAFT_IDLE_MINS", raising=False)
+    monkeypatch.delenv("CRAFT_BUILD_ON", raising=False)
+    monkeypatch.delenv(f"{app_metadata.name.upper()}_BUILD_ON", raising=False)
     with provider_service.instance(
         fake_build_info, work_dir=tmp_path, allow_unstable=allow_unstable
     ) as instance:
@@ -674,6 +676,7 @@ def test_instance(
         mock_provider.launched_environment.assert_called_once_with(
             project_name=fake_project.name,
             project_path=tmp_path,
+            instance_architecture=None,
             instance_name=mock.ANY,
             base_configuration=mock.ANY,
             allow_unstable=allow_unstable,